1. Field of the Invention
The present invention relates generally to aircraft navigation and more particularly to a system employing a Global Positioning System (GPS) and an Inertial Reference System (IRS) to allow for an early determination of when a satellite signal becomes undependable and when a newly acquired satellite signal is undependable.
2. Description of the Prior Art
In the prior art, GPS systems have been used to determine aircraft position by receiving signals from a plurality of satellites. The signals each have information as to the position of the satellites and the time of transmission so that the GPS receiver, on the aircraft, can calculate its own position. Since there are four variables (position in 3 axes and time), signals from at least 4 satellites are necessary for a determination of receiver position. If there are at least five satellites having good geometry each subset of four signals can be used for positioning and they can be compared with each other to determine if one of the signals is in error (Fail safe). If there are at least six satellites in view, then, when there is a faulty signal, it is possible to use the groupings to determine which signal is in error (Fail operational). This procedure has heretofore been accomplished with a system identified as RAIM (Receiver Autonomous Integrity Monitor) which is described in a Honeywell Inc. article titled "Implementation of a RAIM Monitor in a GPS Receiver and an Integrated GPS/IRS" found in a publication entitled Global Positioning System Volume V published by the Institute of Navigation, Alexandria, Va.
Since RAIM cannot detect an erroneous satellite signal with only four satellites or identify which signal is in error with only five satellites, the pilot cannot rely on the position information he receives in these situations and the GPS input must be disregarded. Accordingly, a need has arisen to provide a system which can produce a reliable output when only four satellites are in view.
In a patent application entitled "Navigation System with Solution Separation Apparatus for Detecting Accuracy Failure" Ser. No. 08/712,232, filed Sep. 11, 1996 and assigned to the assignee of the present invention, one solution to this problem is disclosed. More particularly, that application discloses an integrity monitor and positioning algorithm which receives signals from a plurality of remote transmitters, detects failures and determines a protection limit (i.e. the furthest statistical distance by which the position determined by the apparatus will have an almost certain probability of being bounded in a situation where one satellite is in error). This positioning algorithm preferably uses Kalman filtering techniques and incorporates inertial reference data therein to enhance the detection capability. While this system allows confident use of signals from four satellites for a limited time, the protection limit provided by the Kalman filter bank will sometimes exceed the required protection for the most stringent phases of flight (non-precision approach and terminal area navigation) and is therefore not sufficiently available. Furthermore, the bank of Kalman filters is computationally expensive and complex to implement in other equipment than the inertial reference unit. Therefore, a need has arisen for a system which is less complex and expensive, which can more readily be incorporated into equipment of suppliers and which can detect satellite signal errors at an early point in the operation.
In a copending application entitled GPS Signal Fault Isolation Monitor Ser. No. 09/118,046 filed on even date herewith by the present inventor and assigned to the assignee of the present invention, these needs were addressed by providing a new acceleration monitor that produces a first acceleration output along a predetermined axis from the GPS signals and a second acceleration output along the same predetermined axis from the inertial reference units (IRU's).
Although the IRU outputs are not very accurate for determining position, the measurement of acceleration along a predetermined axis is quite accurate. Accordingly, the IRU acceleration signal is compared to the GPS acceleration signal along the predetermined axis. When there is an error greater than a predetermined amount, the faulty satellite signal is identified and can be eliminated from the calculations. Thus only four satellites are necessary for continued operation. In the case of a signal drift, which may be considered as a ramp output, the initial change at the beginning of the ramp causes an acceleration transient which is detected by the acceleration monitor so that early identification of this condition is made. This feature is claimed in the above identified copending application. However, detection of signal drift in a satellite which has begun drifting prior to being seen by the GPS receiver is not detected by the new acceleration monitor since the acceleration transient occurred previously.